binder: fix race between newStream() and unregisterInbound() by synchronizing in-use updates

Previously, concurrent calls to newStream() and unregisterInbound() could both
 update numInUseStreams and invoke transportInUse() in conflicting order,
 leading to inconsistent listener state. This change synchronizes updates and
 only notifies the listener on transitions between 0 and >0.

 Fixes #10917

Author: becomeStar

binder/src/main/java/io/grpc/binder/internal/BinderClientTransport.java

@@ -57,7 +57,7 @@
 import io.grpc.internal.StatsTraceContext;
 import java.util.concurrent.Executor;
 import java.util.concurrent.ScheduledFuture;
-import java.util.concurrent.atomic.AtomicInteger;
+
 import javax.annotation.Nullable;
 import javax.annotation.concurrent.ThreadSafe;
 
@@ -73,7 +73,12 @@ public final class BinderClientTransport extends BinderTransport
   private final Bindable serviceBinding;
 
   /** Number of ongoing calls which keep this transport "in-use". */
-  private final AtomicInteger numInUseStreams;
+  @GuardedBy("this")
+  private int numInUseStreams;
+
+  /** Last in-use state that was reported to the listener */
+  @GuardedBy("this")
+  private boolean listenerInUse;
 
   private final long readyTimeoutMillis;
   private final PingTracker pingTracker;
@@ -114,7 +119,8 @@ public BinderClientTransport(
     Boolean preAuthServerOverride = options.getEagAttributes().get(PRE_AUTH_SERVER_OVERRIDE);
     this.preAuthorizeServer =
         preAuthServerOverride != null ? preAuthServerOverride : factory.preAuthorizeServers;
-    numInUseStreams = new AtomicInteger();
+    numInUseStreams = 0;
+    listenerInUse = false;
     pingTracker = new PingTracker(Ticker.systemTicker(), (id) -> sendPing(id));
 
     serviceBinding =
@@ -259,9 +265,7 @@ public synchronized ClientStream newStream(
       return newFailingClientStream(failure, attributes, headers, tracers);
     }
 
-    if (inbound.countsForInUse() && numInUseStreams.getAndIncrement() == 0) {
-      clientTransportListener.transportInUse(true);
-    }
+    updateInUseStreamsIfNeed(inbound.countsForInUse(), 1);
     Outbound.ClientOutbound outbound =
         new Outbound.ClientOutbound(this, callId, method, headers, statsTraceContext);
     if (method.getType().clientSendsOneMessage()) {
@@ -273,9 +277,7 @@ public synchronized ClientStream newStream(
 
   @Override
   protected void unregisterInbound(Inbound<?> inbound) {
-    if (inbound.countsForInUse() && numInUseStreams.decrementAndGet() == 0) {
-      clientTransportListener.transportInUse(false);
-    }
+    updateInUseStreamsIfNeed(inbound.countsForInUse(), -1);
     super.unregisterInbound(inbound);
   }
 
@@ -305,7 +307,9 @@ void notifyShutdown(Status status) {
   @Override
   @GuardedBy("this")
   void notifyTerminated() {
-    if (numInUseStreams.getAndSet(0) > 0) {
+    if(numInUseStreams > 0) {
+      numInUseStreams = 0;
+      listenerInUse = false;
       clientTransportListener.transportInUse(false);
     }
     if (readyTimeoutFuture != null) {
@@ -391,6 +395,25 @@ private synchronized void handleAuthResult(Throwable t) {
         Status.INTERNAL.withDescription("Could not evaluate SecurityPolicy").withCause(t), true);
   }
 
+  /** Updates in-use-stream count and notifies listener only on transitions between 0 and >0 */
+  private synchronized void updateInUseStreamsIfNeed(boolean countsForInUse, int delta) {
+    if(!countsForInUse) {
+      return;
+    }
+
+    numInUseStreams += delta;
+    if(numInUseStreams < 0) {
+      // Defensive: prevent negative due to unexpected double-decrement
+      numInUseStreams = 0;
+    }
+
+    boolean nowInUseStream = numInUseStreams > 0;
+    if(nowInUseStream != listenerInUse) {
+      listenerInUse = nowInUseStream;
+      clientTransportListener.transportInUse(nowInUseStream);
+    }
+  }
+
   @GuardedBy("this")
   @Override
   protected void handlePingResponse(Parcel parcel) {